Cr In Groundwater Research Articles

Characterisation of the natural attenuation of chromium contamination in the presence of nitrate using isotopic methods. A case study from the Matanza-Riachuelo River basin, Argentina

The groundwater contamination by hexavalent chromium (Cr(VI)) in a site of the Matanza-Riachuelo River basin (MRB), Argentina, has been evaluated by determining the processes that control the natural mobility and attenuation of Cr(VI) in the presence of high nitrate (NO3−) contents. The groundwater Cr(VI) concentrations ranged between 1.9E-5 mM and 0.04 mM, while the NO3− concentrations ranged between 0.5 mM and 3.9 mM.In order to evaluate the natural attenuation of Cr(VI) and NO3− in the MRB groundwater, Cr and N isotopes were measured in these contaminants. In addition, laboratory batch experiments were performed to determine the isotope fractionation (ε) during the reduction of Cr(VI) under denitrifying conditions. While the Cr(VI) reduction rate is not affected by the presence of NO3−, the NO3− attenuation is slower in the presence of Cr(VI). Nevertheless, no significant differences on ε values were observed when testing the absence or presence of each contaminant. The ε53Cr determined in the batch experiments describe a two- stage trend, in which Stage I is characterized by ε53Cr ~−1.8‰ and Stage II by ε53Cr ~−0.9‰. The respective ε15NNO3 obtained is −23.9‰ whereas ε18ONO3 amount to −25.7‰. Using these ε values and a Rayleigh fractionation model we estimate that an average of 60% of the original Cr(VI) is removed from the groundwater of the contaminated site. Moreover, the average degree of NO3− attenuation by denitrification is found to be about 20%. This study provides valuable information about the dynamics of a complex system that can serve as a basis for efficient management of contaminated groundwater in the most populated and industrialized basin of Argentina.

Water Quality Analysis and Health Risk Assessment for Groundwater at Xiangshui, Chongzuo

To investigate the environmental quality and human health risks of different types of groundwater at Xiangshui, Chongzuo, several regular water quality indexes and concentrations of metals in 60 groundwater samples were measured and analyzed. The environmental quality of groundwater was analyzed by means of the Nemerow index. The health risks were assessed by using a human health risk assessment model. The regular water quality indexes and concentrations of metals of the well water, spring water, and underground river water exceeded the standards to different degrees. The environmental quality of groundwater was at a poor grade. The comprehensive evaluation score of underground river water (F=4.26) was the lowest. The well water had the same score as spring water (F=7.10). The high hardness and salinity were conducive to enrichment of Cr, and the reducing environment was of great advantage for the enrichment of As. The environmental geochemistry of Zn, Pb, Cd, and Cu was similar. The sources of Fe, Al, and Mn were similar. The results of the health risk assessment indicated that the health risks of well water, spring water, and underground river water were relatively high. The health risks decreased in the order of well water > underground river water > spring water. The health risks mainly came from the carcinogenic metallic element Cr. Carcinogenic risks were 4-6 orders of magnitude higher than non-carcinogenic risks. Carcinogenic risks were higher than the maximum allowance levels (5.0×10-5 a-1). Non-carcinogenic risks were lower than the allowance levels (10-6 a-1). Children had greater health risks than adults. The health risks of metals through the drinking pathway were 2-3 orders of magnitude higher than the values caused by the dermal contact pathway. For the sake of drinking water safety, the well water, underground river water, and spring water should be properly treated and the concentration of Cr in groundwater should be controlled before drinking.

Spatial diversity of Cr distribution in soil and groundwater sites in relation with land use management in a Mediterranean region: The case of C. Evia and Assopos-Thiva Basins, Greece

The present study compiles new and literature data in a GIS platform aiming to (a) evaluate the extent and magnitude of Cr contamination in a Mediterranean region (Assopos-Thiva and Central Evia (Euboea) Basins, Greece); (b) combine spatial distribution of Cr in soil and groundwater with land use maps; (c) determine geochemical constraints on contamination by Cr; and (d) provide information that will be useful for better management of land use in a Mediterranean type ecosystem in order to prevent further degradation of natural resources.The spatial diversity of Cr distribution in soils and groundwater throughout the C. Evia and Assopos-Thiva Basins is considered. It is attributed to both natural Cr sources (Cr-bearing peridotites, affecting primarily soil) and human (industrial) activities (the dominant source of groundwater contamination).A combination of the spatial distribution of metals in soil and land use maps was used to define the specific areas of agricultural land use with elevated heavy metal contents.Furthermore, the combination of the spatial distribution of Cr in groundwater and land use maps allows for definition of specific areas of industrial land use with elevated Cr concentrations (Inofita, south Assopos-Thiva Basin). Despite the good correlation (r = 0.75) between Cr(VI) and the strong oxidant NO3− in C. Evia, the lower standard potential (E0) values for NO3− compared to those for Cr2O72− (the latter is a stronger oxidant than the former) suggest that NO3− is not an oxidant of Cr.This detailed assessment and presentation of the available analytical data for soil and groundwater in Assopos-Thiva and C. Evia Basins on a land use map provides information for land management decision makers.

Hexavalent Chromium Sources and Distribution in California Groundwater.

Groundwater resources in California represent a confluence of high-risk factors for hexavalent chromium contamination as a result of industrial activities, natural geology, and, potentially, land use. Here, we examine state-wide links in California between groundwater Cr(VI) concentrations and chemicals that provide signatures for source attribution. In environmental monitoring wells, Cr(VI) had the highest co-occurrence and also clustered with 1,4-dioxane and several chlorinated hydrocarbons indicative of the metal plating industry. Additionally, hotspots of Cr(VI) co-occurring with bromoform result from volatile organic compound remediation using in situ chemical oxidation that inadvertently oxidizes naturally occurring Cr(III). In groundwater supply wells, which are typically free of industrial inputs, Cr(VI) correlates with dichlorodiphenyldichloroethylene (DDE), vanadium, and ammonia and clusters with nitrate and dissolved oxygen, suggesting potential links between agricultural activities and Cr(VI). Specific controls on Cr(VI) vary substantially by region: from the metal plating industry around Los Angeles and the San Francisco Bay areas to natural redox conditions along flow paths in the Mojave Desert and to correlations with agricultural practices in the Central Valley of California. While industrial uses of Cr lead to the most acute cases of groundwater Cr(VI) contamination, oxidation of naturally occurring Cr affects a larger area, more wells, and a greater number of people throughout California.

Numerical Modeling of Remediation Scenarios of a Groundwater Cr(VI) Plume in an Alpine Valley Aquifer

This work presents the numerical modeling of remediation scenarios aimed at containing and attenuating the groundwater pollution by Cr(VI) sourced from a steelworks area that affects the Alpine aquifer system in the Aosta Plain (N Italy). Here, groundwater is used for drinking water supply and food and beverage production, so the adoption of remediation works is urgently needed. More specifically, three remediation scenarios were modeled using MODFLOW-2000 and MT3DMS: (a) the activation of a hydraulic barrier to contain the pollution within the source area (Scenario 1); (b) the removal of the pollution sources and the natural attenuation of the residual groundwater plume (Scenario 2); and (c) a combination of the previous two works (Scenario 3). Model results for Scenario 1 showed that a hydraulic barrier composed of five wells located along the eastern border of the steelworks area would contain Cr(VI) concentrations above 5 µg/L (i.e., the Italian regulatory limit) within the steelworks area; the barrier would have a total discharge of 27,500 m3/day, which could be compensated by the deactivation of three steelworks wells; the hydraulic barrier would drop the Cr(VI) concentrations below 5 µg/L in the areas downstream of the steelworks after ~3 years from its start of operation. Results for Scenario 2 highlighted that the removal of the Cr(VI) sources would drop the Cr(VI) concentrations below 5 µg/L in the areas downstream of the steelworks after ~2.5 years, and lead to a full remediation of the Cr(VI) groundwater plume (i.e., total Cr(VI) mass in the aquifer close to zero) after 17 years. Results for Scenario 3 showed that the removal of the Cr(VI) sources accompanied by the activation of the hydraulic barrier would led to a faster remediation within the first 14 years from the starting of the remediation works, with concentrations below 5 µg/L in the areas downstream of the steelworks obtained after ~2.3 years.

Redox transformation and reductive immobilization of Cr(VI) in the Columbia River hyporheic zone sediments

An experimental and modeling study was conducted to investigate the redox transformation and reductive immobilization of groundwater contaminant Cr in hyporheic zone (HZ) sediments from U.S. DOE’s Hanford Site, where groundwater Cr(VI) is migrating and discharging to the nearby Columbia River. Experimental results revealed that Cr(VI) can be reduced and immobilized by the HZ sediments in the presence/absence of O2. Anaerobic pre-incubation of the sediments increased the effective rate of Cr reduction that was correlated with the increase in HCl-extractable Fe(II) content in the sediments. The reduced Cr was stable when exposed to O2 under field-relevant pH (7.5) with and without dissolved Mn(II), which might be oxidized to form Mn(III/IV) oxides that may oxidize reduced Cr. The Cr(VI) reduction rate showed a multi-rate behavior, apparently reflecting the presence of reductants with different reactivity in the sediments. The results from this study indicated that the HZ sediments can reductively immobilize Cr and the sediment redox capacity can be recharged through microbial activities. The results implied that HZ can play a role as a natural permeable redox barrier for removing groundwater Cr before it discharges into a river system.

The fate of Cr(VI) in contaminated aquifers 65 years after the first spillage of plating solutions: A δ53Cr study at four Central European sites

Chromium isotope systematics and concentrations of dissolved Cr were studied in shallow aquifers at four industrial sites in the Czech Republic, Central Europe. Chromium plating baths and wastewaters started to contaminate groundwater in the late 1940s. Since plating shops at all sites still operate, it was possible to determine the δ53Cr signatures of the plating baths. The mean δ53Cr values of the Cr plating baths, representing contamination sources, were 0.0‰ at Zlate Hory (ZH), 0.5‰ at Loucna nad Desnou (LD), 0.3‰ at Letnany (AV), and 0.1‰ at Velesin (JI). At all sites, groundwater Cr was isotopically heavier, compared to the contamination source, reflecting natural in-situ Cr(VI) reduction to insoluble Cr(III). The magnitude of the average positive isotope shift from the plating bath Cr(VI) to groundwater Craq, mostly residual Cr(VI), was 1.1‰, 1.3‰, 3.3‰ and 3.3‰ at ZH, LD, AV, and JI, respectively. The mean concentration of Craq was 0.43, 0.47, 2.38 and 4.32mgL−1, respectively. AV and JI, the sites with lower residual Craq concentrations in groundwater, had higher δ53Cr values, suggesting higher rates/efficiency of Cr(VI) reduction, and of toxicity removal. Simple Rayleigh models indicated that as much as 60% of Cr(VI) has been removed from the groundwater by spontaneous reduction at AV and JI. At ZH and LD, the same models indicated a 30% Cr removal from the solution. Even if uncertainties in model application and Cr fractionation factors are considered, it appears that natural attenuation at Cr(VI)-polluted sites can improve groundwater quality by tens of percent in a relatively short period of time of several decades.

Geochemical constraints on the sources of Cr(VI) contamination in waters of Messapia (Central Evia) Basin

The present study aims to define geochemical constraints on contamination source(s) by Cr(VI) in Central Evia, through a compilation of new and literature data on an integrated set of approaches, including mineralogy and (isotope) geochemistry; leach experiments on rocks, soils and Ni-laterites; and comparison with literature data on groundwater.The concentrations of Cr(VI) in leachates from various geological materials decrease in the following order: Ni laterites from Kastoria > soils > altered ultramafic rocks > Ni laterites from Evia. A salient feature is the diversity in the Cr(VI) concentrations and δ53Cr values in (a) groundwater, ranging from <2 to 850 μg/L Cr(VI) and 0.60–1.99‰ δ53Cr, and (b) leachates from Ni-laterites, ranging from 0.6 to 2.0 μg/L and 1.01‰ in Evia samples and from 750 to 1200 μg/L and negative to slightly positive (−0.21 to 0.03‰) δ53Cr in Kastoria samples. A positive correlation (R2 = 0.514) between the δ53Cr values and depth of the aquifers, potentially reflects the decrease in soluble O2 with increasing depth. The relatively high Cr, Mn and the abundance of fine-grained Mn-(hydr)oxides in the Kastoria laterite ore, may be major controlling factors for the existence of high Cr(VI) concentrations in the water leachates. In contrast, the low Cr(VI) concentrations and positively fractionated Cr isotopes in the Ni-laterite leachates from Evia are potentially related to redox and multistage transportation/redeposition processes.The positive correlation between Cr concentrations and Ca/Mg ratios (R2 = 0.647) for rock leachates and the negative correlation (R2 = −0.694) for soil leachates suggest that Cr in groundwater may be released mainly from Cr-bearing Mg-silicates in altered ultramafic rocks and soils. The plot of δ53Cr values versus Cr(VI) concentrations for the contaminated waters from Central Evia and Assopos Basin fall within both fields defined from a global database for natural waters and waters contaminated by human activities.

Assessment of trace metal concentration and health risk of artisanal gold mining activities in Ijeshaland, Osun State Nigeria— Part 1

This present study assessed the trace metal concentrations and human health risks associated with the activities of artisanal gold miners in Ijeshaland, Osun State, Southwestern Nigeria. Environmental samples, including surface water (rivers and streams), groundwater, bottom sediments, fish (Tilapia) and floating plants (ferns) were collected between 2011 and 2012 and analyzed for important parameters using standard procedures. Fifty four environmental samples were collected and analyzed for essential and nonessential trace metals. Data collected were subjected to simple descriptive and inferential statistics. Health risk indexes of noncarcinogenic and carcinogenic adverse effects of trace metals were determined in water and fish samples. Results of metal concentrations revealed Al (305±950μgL−1) and Fe (0.31±0.56mgL−1) in surface water higher than the World Health Organization (WHO) permissible limits in drinking water. The hazard quotient (HQ) values of Al and Fe (from surface and ground water), Mn (from groundwater) and Pb (from fish) were higher than 1.0 indicating noncarcinogenic adverse effects. The cancer risk (CR) values of Cd and Cr in groundwater, surface water and fish samples ranged between 7.9×10−4 and 1.0×10−2 thereby establishing their carcinogenic effects. The CR values of Ni and Pb in water consumed by infants and children were also higher than the acceptable limits of 1.0×10−4. The results allow us to assert that the activities of the artisanal gold miners clearly affect human health.

Coupled Hydro-Biogeochemical Processes Controlling Cr Reductive Immobilization in Columbia River Hyporheic Zone.

An experiment and modeling study was conducted to investigate coupled hydro-biogeochemical processes controlling reductive immobilization of groundwater Cr in the hyporheic zone (HZ) at the U.S. Department of Energy's Hanford Site, where dynamic surface water-groundwater exchange occurs on a daily basis. Experiments were performed to calibrate kinetic models, and the calibrated models were incorporated into a multicomponent reactive transport model to simulate Cr redox transformation and immobilization under field hydrological conditions. The results revealed that the rates of Cr(VI) reduction, Cr(III) accumulation, and Cr(VI) release to the river are mostly affected by dynamic sediment redox conditions represented by Fe(II) reactivity, which is controlled by its cyclic interaction with O2 carried by river water, microbial activities, and the supply and bioavailability of organic carbon (OC) that is present in the HZ and/or carried by transport. In addition, the HZ geophysical properties including hydraulic conductivity and the thickness of the top alluvial layer have a significant influence on Cr reactive transport and immobilization by controlling residence times for reactions and the supply rates of O2, Cr, and OC into the HZ. The results provide important insights into the dynamic redox environments in the HZ that can reductively immobilize contaminants.

Origin of Hexavalent Chromium in Drinking Water Wells from the Piedmont Aquifers of North Carolina

Hexavalent chromium [Cr(VI)] is a known pulmonary carcinogen. Recent detection of Cr(VI) in drinking water wells in North Carolina has raised public concern about contamination of drinking water wells by nearby coal ash ponds. Here we report, for the first time, the prevalence of Cr and Cr(VI) in drinking water wells from the Piedmont region of central North Carolina, combined with a geochemical analysis to determine the source of the elevated Cr(VI) levels. We show that Cr(VI) is the predominant species of dissolved Cr in groundwater and elevated levels of Cr and Cr(VI) are found in wells located both near and far (>30 km) from coal ash ponds. The geochemical characteristics, including the overall chemistry, boron to chromium ratios, and strontium isotope (87Sr/86Sr) variations in groundwater with elevated Cr(IV) levels, are different from those of coal ash leachates. Alternatively, the groundwater chemistry and Sr isotope variations are consistent with water–rock interactions as the major source for Cr(...

Metal pollution in water environment and the associated human health risk from drinking water: A case study of Sukinda chromite mine, India

ABSTRACTSukinda is one of the most polluted regions of the globe due to chromium pollution. The study sheds light into the heavy metal pollution around the mining area and its effect on the health of the resident populace. The Cr(VI) was in the range of 0.027–2.48 mg/L in surface water and BDL–1.35 mg/L in the groundwater. Multivariate analysis revealed that mining activity was the main source of TCr, Cr(VI), and Zn in the surface water that warrants attention. Heavy metal evaluation index showed high levels of Fe, Mn, and Cr in groundwater that can pose serious threat to the exposed population. Cancer and non-cancer risk of Cr(VI) was higher than other metals in groundwater. The results revealed that the total cancer risk was 1.21E-03 and 1.05E-03 in adults and children, respectively, which exceeded the USEPA acceptable cancer health risk. High health risk was observed through oral intake of water, while both cancer and non-cancer risks were negligible through dermal contact. This study strongly advocates proper periodic assessment of drinking as well as surface water in the area and regulation to restrict the use of contaminated water for daily use.

High Nitrate Content in the Surface Water of Balipara, North Brahmaputra River Basin, Sonitpur District, Assam, India:A Multivariate Approach

The present study is an evaluation of ground and surface water quality at Balipara, North Brahmaputra river basin, Sonitpur district, Assam, India using multivariate statistical methods. The results show high concentration of Fe, Mn, Pb and Cr in groundwater. Arsenic was observed in both ground and surface water. In the surface water, nitrate content was also found to be high. Ward's method was used for hierarchical agglomerative cluster analysis. A close relationship between electrical conductivity (EC) and total dissolved solids (TDS) was established by the smallest proximity distance between these parameters. Group 1 comprised of TDS, EC, total alkalinity (TA), F, Ca, Pb, Cr and Cl based on proximity distances. Group 2 consisted of Fe, Mn, As and Group 3 of TH, Mg, pH, Zn, SO 4 and NO 3 in groundwater. In surface water, Group 1 comprised of TDS, EC, SO4, NO3, Cl, Zn, pH and Ca. Group 2 accounted for Mg and F and Group 3 for Fe, Mn, TA, Pb, As and Cr. The interrelationships between the contaminants depicted by cluster analysis, categorize the contamination levels. Factor analyses were applied for understanding the interrelationships between the variables and for identifying probable source components. Six factors justifying 83.64% of the total variance in groundwater and five factors describing 81.92% of the total variance in surface water were found responsible for variation in the data structure. The relative contribution of all the water- quality parameters was best explained by discriminant analysis.

Biological treatment of groundwater with a high hexavalent chromium content under anaerobic and anoxic conditions

AbstractBACKGROUNDMicrobial reduction of Cr(VI) to the much less toxic Cr(III), although practiced for the treatment of liquid wastes has not been implemented for the treatment of groundwater due to the absence of electron donors (organic substrates) and the lack of data on inhibition of biological Cr(VI) removal. The objective of this work is to evaluate biological groundwater treatment systems that will achieve high Cr(VI) removal with addition of a low dose of organic substrate and to estimate inhibition of microbial Cr(VI) reduction under anaerobic and anoxic conditions.RESULTSBiomass acclimatized to Cr(VI) under anaerobic conditions exhibited a higher sensitivity to Cr(VI) compared with biomass acclimatized to Cr(VI) under anoxic conditions. No significant inhibition of microbial growth was observed under anaerobic conditions for initial groundwater Cr(VI) concentrations up to 2000 µg L−1, whereas no significant inhibition was obtained under anoxic conditions at Cr(VI) concentrations as high as 10 000 µg L−1.CONCLUSIONThis study demonstrates that anoxic biological systems treating groundwater can provide complete hexavalent chromium removal at initial hexavalent chromium concentrations as high as 10 000 µg L−1. In addition anaerobic conditions can support complete hexavalent chromium removal at initial hexavalent chromium concentrations as high as 2000 µg L−1. © 2016 Society of Chemical Industry

A Slow-Release Substrate Stimulates Groundwater Microbial Communities for Long-Term in Situ Cr(VI) Reduction.

Cr(VI) is a widespread environmental contaminant that is highly toxic and soluble. Previous work indicated that a one-time amendment of polylactate hydrogen-release compound (HRC) reduced groundwater Cr(VI) concentrations for >3.5 years at a contaminated aquifer; however, microbial communities responsible for Cr(VI) reduction are poorly understood. In this study, we hypothesized that HRC amendment would significantly change the composition and structure of groundwater microbial communities, and that the abundance of key functional genes involved in HRC degradation and electron acceptor reduction would increase long-term in response to this slowly degrading, complex substrate. To test these hypotheses, groundwater microbial communities were monitored after HRC amendment for >1 year using a comprehensive functional gene microarray. The results showed that the overall functional composition and structure of groundwater microbial communities underwent sequential shifts after HRC amendment. Particularly, the abundance of functional genes involved in acetate oxidation, denitrification, dissimilatory nitrate reduction, metal reduction, and sulfate reduction significantly increased. The overall community dynamics was significantly correlated with changes in groundwater concentrations of microbial biomass, acetate, NO3-, Cr(VI), Fe(II) and SO4(2-). Our results suggest that HRC amendment primarily stimulated key functional processes associated with HRC degradation and reduction of multiple electron acceptors in the aquifer toward long-term Cr(VI) reduction.

Assessment of heavy metals concentrations with emphasis on arsenic in the Tabriz plain aquifers, Iran

The Tabriz area is one of the most industrialized areas in northwest of Iran. The heavy industrialization and the increasing urbanization are responsible for the rapidly increasing stress on the groundwater of the area. The objective of this study was to evaluate heavy metals such as Fe, Mn, Al, Zn, As and Cr in groundwater and their spatial distribution in Tabriz plain aquifer. The concentrations of Fe, Cr, Mn, As, and Al in some of the groundwater samples exceed the maximum admissible concentration (MAC). Correlation matrix (CM) and principal components analysis (PCA) methods were employed to identify source apportionment of contamination parameters in groundwater. There are two types of aquifers, with different quality of water, in the plain; an unconfined aquifer with saline water and a confined aquifer with fresh water quality. Arsenic concentrations in the unconfined aquifer and in the recharge areas of the plain boundary are low, with an average of 25.8 µg/L, while in the confined aquifer and deep wells the concentrations are high, with an average of 122.5 µg/L. The arsenic occurring in groundwater resources of the area originates from geological formations such as alluvial tuffs of Sahand mountain and its concentration is highly dependent on hydrogeological and environmental reducing conditions, residence time of water and depth of the sampling wells.

Distribution of heavy metals (Cu, Zn and Cr) in groundwater from the area of a future radioactive waste repository Saligny – Romania

A study of some heavy metals (Cu, Zn and Cr) concentrations in natural groundwater has been conducted, by considering samples collected from the area located in the neighbourhood of Saligny village (Cernavoda, Romania). Atomic absorption spectrometry methods with thermal and electrothermal atomization has been developed, tested and used for some heavy metals content determination. The results show various concentrations of Cu, Zn and Cr in groundwater samples.

A chromate-contaminated site in southern Switzerland – Part 2: Reactive transport modeling to optimize remediation options

► Development of a 2D reactive transport model of a Cr(VI) contaminated site. ► Model incorporation of site mineralogy, Cr isotopes, water chemistry and site hydrology. ► Simulation of measured present day Cr(VI) concentrations and δ 53 Cr values. ► Prediction of Cr(VI) plumes for alternate remediation strategies. ► Prediction of remaining time with ongoing subsoil Cr(VI) contamination. A 2D horizontal reactive transport model of a chromate-contaminated site near Rivera, Switzerland, was developed using the computer code CrunchFlow to evaluate site remediation strategies. Transport processes were defined according to the results of an existing hydrological model, and the definition of geochemical (reactive) processes is based on the results of a detailed mineralogical and geochemical site characterization leading to a comprehensive conceptual site model. Kinetics of naturally occurring Cr(VI) reduction by Fe(II) and natural solid organic matter is quantified by fitting measured Cr isotope ratios to a modeled 1D section along the best constrained flow line. The simulation of Cr isotope fractionation was also incorporated into the 2D model. Simulation of the measured present day Cr(VI) plume and δ 53 Cr value distribution was used for the 2D model calibration and corresponds to a situation where only monitored natural attenuation (MNA) is occurring. Other 2D model runs simulate alternate excavation scenarios. The simulations show that with an excavation of the top 2–4 m the groundwater Cr(VI) plume can be minimized, and that a deeper excavation depth only diminishes the plume if all the contaminants can be removed. A combination of an excavation of the top 2–4 m and monitoring of the ongoing natural Cr(VI) reduction is suggested as the most ecological and economical remediation strategy, even though a remaining time period with ongoing subsoil Cr(VI) contamination in the order of 1 ka is predicted.

A chromate-contaminated site in southern Switzerland – Part 1: Site characterization and the use of Cr isotopes to delineate fate and transport

Abstract The risk of groundwater contamination by chromate at a former chromite ore processing industrial site in Rivera (Switzerland) was assessed by determining subsoil Cr(VI) concentrations and tracking naturally occurring Cr(VI) reduction with Cr isotopes. Using a hot alkaline extraction procedure, a total Cr(VI) contamination of several 1000 kg was estimated. Jarosite, KFe 3 ((SO 4 ) x (CrO 4 ) 1− x ) 2 (OH) 6 , and chromatite (CaCrO 4 ) were identified as Cr(VI) bearing mineral phases using XRD, both limiting groundwater Cr(VI) concentrations. To track assumed Cr(VI) reduction at field scale δ 53 Cr values of contaminated subsoil samples in addition to groundwater δ 53 Cr data are used for the first time. The measurements showed a fractionation of groundwater δ 53 Cr values towards positive values and subsoil δ 53 Cr towards negative values confirming reduction of soluble Cr(VI) to insoluble Cr(III). Using a Rayleigh fractionation model, a current Cr(VI) reduction efficiency of approximately 31% along a 120 m long flow path was estimated at an average linear groundwater velocity of 3.3 m/d. Groundwater and subsoil δ 53 Cr values were compared with a site specific Rayleigh fractionation model proposing that subsoil δ 53 Cr values can possibly be used to track previous higher Cr(VI) reduction efficiency during the period of industrial activity. The findings strongly favor monitored natural attenuation to be part of the required site remediation measures.

Geochemistry of natural chromium occurrence in a sandstone aquifer in Bauru Basin, São Paulo State, Brazil

Anomalous concentrations of Cr(VI) occur in groundwaters of the Adamantina Aquifer, in a large region in the western state of São Paulo, sometimes exceeding the potability limit (0.05 mg L−1). To identify the possible geochemical reactions responsible for the occurrence of Cr in groundwater in Urânia, borehole rock samples were collected in order to carry out mineralogical and chemical analyses. In addition, multilevel monitoring wells were installed and groundwater samples were analyzed. Analyses of the borehole rock samples show the occurrence of a geochemical anomaly of Cr in the quartzose sandstones (average concentrations of 221 ppm). Chrome-diopside is one of the main minerals contributing to this anomaly, having an average Cr content of 1505 ppm. Sequential extraction experiments indicated weakly adsorbed Cr in the order of 0.54 ppm, and this quantity is enough to provide the Cr concentrations observed in groundwater. Groundwaters from the monitoring wells proved to be stratified, with the highest concentrations of Cr(VI) (0.13 mg L−1) being associated with high redox and pH values (over 10) and high concentrations of Na. Geochemical reactions that may explain the release of Cr from the solid phase to groundwater involve the release of Cr(III) from minerals (like chrome-diopside and Cr–Fe hydroxide), followed by oxidation of Cr(III) to Cr(VI), probably related to the reduction of Mn oxides present in the aquifer. Then cation exchange occurs and dissolution of carbonates which increases the pH of groundwater, resulting in the desorption and mobilization of Cr(VI) into groundwater.